1. Field of the Invention
This invention relates, generally, to apparatus and methods of operation for a filter bed installed in a microbial remediation reactor or in other chemical, petrochemical, waste, or food process vessels. More specifically, the invention relates to improvement of filter bed performance by moving and/or preventing formation of the sludge mat that tends to settle on top of the filter bed surface, so that the sludge mat does not block flow through the filter bed to an extent that requires premature backwashing of the filter bed.
2. Background Art
The instant invention is particularly effective in, but is not limited to, downflow sludge blanket reactors used for microbial remediation of various wastewater streams. For example, the instant invention may be effective for treating streams comprising activated sludge; fats, oils, and grease (“FOG”); suspended BOD solids; and soluble BOD; such as flow from municipal waste plants or food and meat processing plants. The invention is particularly effective in optimizing anaerobic microbial treatment of high FOG and high total suspended solids (“TSS”) streams, or treatment of lower FOG/TSS streams or soluble BOD streams wherein low residence times and the associated high effluent flowrates require efficient filter bed performance and control of filter bed plugging. Examples of some of the many embodiments of microbial reactors that may benefit from the instant invention are described in patents to Stormo (U.S. Pat. No. 5,616,304, “Slurry Reactor”; U.S. Pat. No. 5,744,105, “Slurry Reactor”; and U.S. Pat. No. 5,779,996, “Microbial Remediation Reactor and Process”; and U.S. Pat. No. 6,346,412, “Microbial Remediation Reactor and Process”).
The preferred microbial reactor may utilize a diverse population of suspended microorganisms (herein “biomass”) to treat the waste and contaminants contained in the mixed liquor in the reactor (the bulk liquid in the reactor containing suspended and dissolved solids). In the bottom of the preferred downflow reactors is a settled bed of coarse solids, such as course sand. The sand bed filters reactor liquid as it passes through the filter bed, and out of the reactor as reactor effluent. The sand bed filters out the solids (including biomass and other solids) from the reactor liquid, in order to retain the biomass in the reactor and to retain the waste solids in the reactor for the desired remediation residence time. Without preventive or remediative measures, the sand bed typically becomes plugged with solids, and, in doing so, slows and eventually stops the flow of reactor effluent. When flow through the filter bed slows, the conventional solution is to “backwash” the filter bed, by the conventional technique of liquid flow up through the filter bed, or by the technique of stirring and fluidizing the filter bed as discussed below.
U.S. Pat. No. 6,346,412 (“the '412 patent”) teaches the use of a mixing blade P-78 inside the sand bed P-84 as a means to prevent or alleviate plugging of the sand bed. See FIG. 1. Patent '412 teaches that the sand bed “may be used as a filter as the water is removed from the bottom of the reactor. Any suspended material, including biomass, is retained by the sand filter therefore allowing very high biomass densities to be maintained, with resulting very high activity. As the mixing blade moves through the sand bed, it fluidizes the sand near the blade and keeps the biomass from plugging the sand or the screened outlet.” See screen P-86 and outlet P-82 in FIG. 1. Thus, the mixing blade in the sand bed as described in U.S. Pat. No. 6,346,412 performs a “backwash” step, of unplugging or preventing plugging of the sand bed, by rotating through the sand bed to fluidize the sand bed. While this stirring technique does not involve back-washing with up-flow liquid and does not involve emptying the reactor, it does require shutting off the flow out of the reactor, and, hence, is disruptive to the reactor operation and productivity.
The '412 patent further discloses the use of a sand bed mixing blade in an aerobic reactor, wherein the mixing blade is in the upper region of the sand bed, while leaving the lower region of the sand bed relatively undisturbed. This technique is a means for back-washing the sand bed while leaving the lower region of the sand bed as an anaerobic denitrification region. Still, this technique involves stirring of the sand bed to solve the low/no reactor effluent problem in the reactor.
To rotate a sand bed stirring mechanism, such as embodiments described in the '412 patent, the horizontal stirrer blade P-78 is supplied with fluid from a central, vertical supply conduit P-76. Fluid preferably passes through openings at or near the blade's leading side for fluidization, and through openings at or near the blade's trailing side for propulsion. The fluidization back-washes the sand, removing solids from the voids in the bed and forcing the solids back up into the reactor. After this back-washing process is performed, movement of the stirring mechanism is preferably stopped and flow of reactor effluent resumes.
In some microbial remediation reactor operations, the effluent flow-rate can easily be maintained with intermittent fluidization by means of the above-described in-sand stirrer blade “back-washing” the sand bed. However, the inventors have discovered that in many operations and under many conditions, the frequency of sand bed backwashing must increase to an extent that makes it difficult to maintain productive operation of the reactor, and difficult or impossible to retain sufficient biomass in the reactor for the desired microbial activity and to retain the waste solids in the reactor for the residence time required for remediation.
Therefore, there is still a need for improved apparatus and methods for operating a reactor having a filter bed in such a way that frequent backwashing of the filter bed is not required to maintain an acceptable rate and quality of reactor effluent discharge. There is a need for apparatus and methods that improve filter bed operation, especially in moderate BOD applications such as waste-waters having 1000 mg/L BOD and 100 TSS up to 50,000 mg/L BOD and 10,000 TSS. Such applications are characterized by short residence times on the order of hours, rather than days, and consequently the reactors are designed for high effluent flowrates. The filter beds of such applications need to be effective and long-lived in order to achieve these flowrates. The present invention meets these and other needs, as will be clear from the following description, drawings, and claims.